L-aspartic acid, often just called aspartic acid, is an amino acid — the building block of proteins and necessary for several other processes in the body. For instance, you can burn amino acids for energy, convert them into fat and other molecules, or use them to make your own cellular proteins.
Amino acids are small carbon-based molecules that your cells require for a number of purposes. You obtain them whenever you eat protein; enzymes in the stomach and in your small intestine break down dietary proteins into their constituent amino acids, and you absorb them into the bloodstream and eventually into the cells. There are 20 "normal" amino acids, plus a few modified amino acids — of which L-aspartic acid is one.
All amino acids have three of the same groups bonded to a central carbon: a "COOH" group, an "NH3" group and a hydrogen. They also each have a distinct sidechain; aspartic acid's is "CH2COOH." The reason for the "L" in front of aspartic acid's name is that technically, aspartic acid — like most of the other amino acids — is asymmetric, and exists in two mirror-image forms. Only one form, denoted by the L, occurs naturally in proteins, however.
Like all amino acids, your body can use aspartic acid to provide your cells with energy; the cells burn it to generate ATP, or adenosine triphosphate, which is a cellular energy currency. While aspartic acid is useful, it's not essential in the human diet — your body makes it from a molecule called oxaloacetate, which you produce any time you metabolize fuel, regardless of whether the fuel is carbohydrate, protein or fat.
There are a few things your body can do with aspartic acid that no other amino can perform. It stimulates a neural receptor called the NMDA receptor, which plays a role in memory and cognition. You can also use it to make several other amino acids, making it useful for preventing amino acid deficiencies. Finally, it has an important role in gluconeogenesis, which is the process of making glucose, or sugar, when your supply is low.